On-chip packet-switched communication system

ABSTRACT

A system for routing a data packet between N elements includes N network interfaces respectively connected to the N elements, with N being an even integer, and an on-chip packet-switched communication network arranged in a ring structure. The packet-switched communication network includes N routers respectively connected to the N interfaces, and N pairs of opposite uni-directional ring links. Each pair of ring links couples two adjacent routers in the ring structure, and each ring link provides two virtual channels. There are N/2 pairs of opposite uni-directional crossing links, with each pair of crossing links coupling two diametrically opposite routers in the ring structure. Processing circuitry is distributed within the N routers and the N network interfaces for determining direction of the data packet to be transmitted over a path from a source element to a destination element in the ring structure, and for determining at each router in the path which virtual channel is to be used to avoid deadlocks in the transmission

FIELD OF THE INVENTION

The present invention relates in general to on-chip communicationnetworks, and more particularly, to a packet-switched network.

BACKGROUND OF THE INVENTION

There are two fundamental approaches toward building a network core:circuit switching and packet switching. In circuit-switched networks,the resources needed along a path (buffers, links) for providingcommunication between the end systems are reserved for the duration ofthe communication session.

European Patent No. 1,257,100 discloses a circuit-switched network,partly scalable, with a number of the nodes being a multiple of eight.More precisely, eight nodes form a ring. It is necessary to connectseveral different rings with each other when more than eight nodes(still multiple of eight) are to be used

In packet-switched networks, the resources are not reserved. Packets usethe resources on demand, and as a consequence, may not have to wait foraccess to a communication link. However, in conventional packet-switchednetworks, problems exist, such as a lack of scalability and deadlock.

Deadlock is a situation where a group of data packets are unable to makeprogress because they are waiting on one another. In other words, a datapacket is unable to make progress because other data packets blockresources which are necessary for the data packet to progress toward itsdestination.

Approaches exist, including a general purging of resources, but thiscauses a loss of data. Moreover, the current shared bus systems showtheir limitations in terms of performance, scalability, powerconsumption and reliability.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an on-chipcommunication architecture based on micro-networks implementing aflexible packet-switched communication, which is free of deadlock.

Another object of the present invention is to provide a scalablepacket-switched system having a low cost, even if the number of nodes islarge, such as more than eight, for example.

The system for routing data packets between elements comprises N networkinterfaces respectively adapted to be connected to N elements, with Nbeing an even integer, and an on chip packet-switched communicationnetwork is arranged in a unique ring structure.

The ring structure includes N routers respectively connected to the Ninterfaces, and N pairs of opposite uni-directional ring links. Eachring link may carry two virtual channels, and one pair of ring links maycouple two adjacent routers on the ring. The ring structure may alsoinclude N/2 pairs of opposite uni-directional crossing links, with onepair of crossing links coupling two diametrically opposite routers ofthe ring.

The system may further comprise processing means or circuitry,distributed within the routers and the network interfaces, and adaptedupon the presence of a data packet to be transmitted from a sourceelement to a destination element. This determines the directioncharacteristics of the path of the data packet on the ring, and ifnecessary, to determine at each router belonging to the path whichvirtual channel is to be used so as to avoid deadlocks in thetransmission.

The system thus combines a unique ring regardless of the value of N, twovirtual channels per each ring link, and an appropriate determination ofthe path within the ring structure using the virtual channels. Ofcourse, the ring according to the invention is not necessarily circularand can have, once implemented on the chip, any geometrical form.

According to a preferred embodiment of the invention, which permits anydeadlock to be easily avoided, the processing is adapted to define thepath for the transmitted data packet, such that the path includes atleast one transition on the ring between two adjacent routers, or acrossing transition between two diametrically opposite routers followedby a circular path on the ring having at least one transition. Thedirection of a circular path is identical for all the transitions of thecircular path.

A circular path is a path on the ring, regardless of the geometricalform of the ring. The choice between the two virtual channels, ifnecessary, is a function of the index of the router on the ring, forexample.

According to a preferred embodiment, the processing may be adapted todefine the path for the transmitted data packet, such that the pathcomprises at the most d transitions between routers, with d being equalto ceil (N/4).

According to one embodiment, a data packet may comprise a headerincluding a first field defining the presence of a crossing transitionand/or the direction of a circular path on the ring, and a second fielddefining the destination router. The distributed processing compriseslocal preprocessing modules in the network interfaces. The localpreprocessing module of the network interface connected to the sourceelement is adapted for determining the content of the first field andthe second field. The processing may also comprise a local router withinthe routers for routing such data packets in accordance with the contentof the first and second fields.

In a preferred embodiment, the successive routers may be indexed withconsecutive indexes, and the local preprocessing module of the sourcenetwork interface may comprise a calculating module for calculating avalue r equal to the difference modulo N between the index of thedestination router and the index of the source router, and a determiningmodule for determining the content of the first field from the value ofr.

Advantageously, the indexes may be increased in the right or clockwisedirection, and the determining module may be adapted to provide thecontent of the first field with a plurality of different values. Thedifferent values are discussed below.

A first value is representative of the right or clockwise direction onthe ring if the value r belongs to the set {1,2, . . . ,d}. A secondvalue is representative of the left or counter-clockwise direction onthe ring if the value r belongs to the set {N−d,N−d+1, . . . ,N−1}. Athird value is representative of the crossing transition between twodiametrically opposite routers if the value r is equal to N/2.

A fourth value is representative of the crossing transition between twodiametrically opposite routers followed by the left or counter-clockwisedirection on the ring if the value r belongs to the set {d+1,d+2 . . .,N/2−1}. The fifth value is representative of the crossing transitionbetween two diametrically opposite routers followed by the right orclockwise direction on the ring otherwise if the value r belongs to theset {N/2+1,N/2+2, . . . ,N−d−1}.

Furthermore, each router may be adapted for routing the data packet toits coupled network interface if the index of the corresponding routeris equal to the content of the second field of the header of the datapacket. Otherwise, each router may route the data packet according tothe content of the first field of the header of the data packet on afirst virtual channel if the index is greater than the content of thesecond field of the header of the data packet, and on a second virtualchannel otherwise.

Each router may comprise at least two queues connected to auni-directional ring link connected to the router. Each virtual channelmay be defined by at least one queue and the correspondinguni-directional ring link.

Advantageously, indexes may be binary encoded, and the routers may beadapted to process indexes with the last ceil(log₂(d)) bits of theindexes. Reducing the size of this data optimizes the use of thebandwidth of the network, and the processing time.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will appear on examiningthe detailed description of embodiments, which are in no way limiting,and of the appended drawings in which:

FIG. 1 illustrates a system according to the present invention;

FIG. 2 illustrates routers with virtual channels for the system shown inFIG. 1;

FIG. 3 a illustrates a network interface for a system according to thepresent invention;

FIG. 3 b illustrates a data packet according to the present invention;

FIG. 4 illustrates a first path for a packet according to the presentinvention;

FIG. 5 illustrates a second path for a packet according to the presentinvention;

FIG. 6 illustrates a third path for a packet according to the presentinvention;

FIG. 7 illustrates a fourth path for a packet according to the presentinvention; and

FIG. 8 illustrates a fifth path for a packet according to an aspect ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, the illustrated system 1 is incorporated on achip IC. The system comprises N=10 routers r₀,r₁, . . . ,r_(N-1) indexedfrom 0 to N−1=9 in the clockwise or right direction. Each of the routersr₀,r₁, . . . ,r_(N-1) is connected to a respective network interfaceNI₀,NI₁, . . . ,NI_(N-1). Each of the network interfaces NI₀,NI₁, . . .,NI_(N-1) is connected to a corresponding element E₀,E₁, . . . ,E_(N-1).Elements E₀,E₁, . . . ,E_(N-1), can be, for example, a processor, amemory, an input, an output, a sub-system bus, or any other element.

The routers r₀,r₁, . . . ,r_(N-1) form a unique ring RG. Two adjacentrouters r_(i),r_(j) of the ring RG are linked with a pair of oppositeuni-directional ring links rl_(ij) (from r_(i) to r_(j)) and rl_(ji)(from r_(j) to r_(i)) Each ring link rl_(kl) carries two virtualchannels vl1 _(kl), vl2 _(kl).

More precisely, FIG. 2 represents three adjacent routersr_(i),r_(j),r_(k) of the ring RG. Each router of the system comprises arouting circuit rmi. Each router further comprises at least two queuesq1 _(ij), q2 _(ij) and a multiplexer Mux_(ij) connected to theuni-directional ring link for transmitting a data packet from a routerr_(i) to a router r_(j). The multiplexer Mux_(ij) is controlled by therouting circuit rm_(i). Each of the at least two queues q1 _(ij), q2_(ij) permits definition of a virtual channel associated with theuni-directional ring link. Each virtual channel has at least one queue,and each uni-directional ring link rl_(ij) has two virtual channels vl1_(ij), vl2 _(ij).

Referring back to FIG. 1, two routers r_(i) and r_(j), diametricallyopposite each other (j-i) mod N=N/2) on the ring RG are linked with apair of opposite uni-directional crossing links cl_(ij) (from r_(i) tor_(j)) and cl_(ji) (from r_(j) to r_(i)).

In FIG. 3 a, a network interface NI_(i) is illustrated in greaterdetail. The system comprises processing circuitry, which is distributedwithin the routers r₀,r₁, . . . ,r_(N-1), and the network interfacesNI₀,N₁, . . . ,NI_(N-1). More precisely, the processing circuitrycomprises N identical local preprocessing modules ppm₀,ppm₁, . . .,ppm_(N-1) located within the N network interfaces NI₀,NI₁, . . .,NI_(N-1), and N identical routing modules rm₀,rm₁, . . . ,rm_(N-1)located within the N routers r₀,r₁, . . . ,r_(N-1). The processingcircuitry is adapted to determine the path of a data packet P from asource element E_(source) to a destination element E_(destination) ofthe system.

The path comprises at most d transitions between routers, with d beingequal to ceil(N/4). The ceil function of a variable x, when y−1<x<y, isequal to y, with x being a real number and y being an integer number.

As illustrated in FIG. 3 b, a data packet P comprises a header H havinga first field f₁ and a second field f₂. The network interface NI_(i) ofa source element E_(source) comprises a preprocessing moduleppm_(source) which is capable of determining the content of the twofields f₁,f₂ of the header H of the data packet P for a data packet P tobe transmitted. The first field f₁ defines the presence of a crossingtransition and/or the direction of a circular path on the ring RG. Onceon the ring, the data packet P keeps the same sense clockwise orcounter-clockwise. The second field f₂ contains the index of thedestination router r_(destination) corresponding to the destinationnetwork interface NI_(destination) and the destination elementE_(destination) with its index.

Preprocessing modules ppm₀,ppm₁, . . . ,ppm_(N-1) use a specific processto determine the content of the two fields f₁,f₂ of the header H of thedata packet P to be transmitted from a source network interfaceNI_(source) to a destination network interface NI_(destination). Eachpreprocessing means ppm_(i) comprises a calculating module Calc_(i)which calculates a value r equal to the difference between the index ofthe destination router and the index of the source router modulo N:r=(destination−source) modulo N.

Furthermore, each preprocessing module ppm_(i) comprises a determiningmodule Det_(i) for determining the content of the first field f₁ fromthe value of r. More precisely:

If r belongs to the set {1,2, . . . ,d}, a first value R representativeof the right or clockwise direction on the ring RG is set to the firstfield f₁.

If r belongs to the set {N−d,N−d+1, . . . ,N−1}, a second value Lrepresentative of the left or counter-clockwise direction on the ring RGis set to the first field f₁.

If r is equal to N/2, a third value A representative of the crossingring direction RG is set to the first field f₁.

If r belongs to the set {d+1,d+2 . . . ,N/2−1}, a fourth value (AL)representative of the crossing ring direction followed by the left orcounter-clockwise direction on the ring RG is set to the first field f₁.

A fifth value AR representative of the crossing ring direction followedby the right or clockwise direction on the ring is otherwise set to thefirst field f₁, i.e., if r belongs to the set {N/2+1,N/2+2, . . .,N−d−1}.

In the present example, the direction (R,L) corresponds to theincreasing indexation direction of the routers r₀,r₁, . . . ,r_(N-1). Ifthe indexation direction is opposite, the content of the field f₁ hasthe opposite value.

There are five possible values for the content of the first field f₁.Theoretically, three bits are necessary, but it is advantageouslypossible to encode the first field f₁ on 2 bits. This is because thecase of the third value A can be joined either with the case of thefourth value AL, or with the case of the fifth value AR. In case A,after the crossing transition, the data packet P will be directly sendto the corresponding network interface.

An example of an algorithm illustrating this method, wherein case A(only a crossing transition) is joined with case AL (a crossingtransition followed by at least one transition in the left orcounter-clockwise direction on the ring RG) is shown below: r

(destination − source) mod N; if ( r ∈ {1,2,3,...,d} )  then   f₁

R;  else   if ( r ∈ {N−1,N−2,N−3,...,N−d} )    then    f₁

L;   else    if ( r ∈ {d+1,d+2,...,N/2} )     then      f₁

AL;     else      f₁

AR;    endif;  endif; endif;

Each routing module rm_(i) is capable of routing the data packet to itscoupled network interface if the index i of the corresponding router isequal to the content (destination) of the second field f₂ of the headerH of the data packet P. Otherwise, if its index i is greater than thecontent (destination) of the second field f₂ of the header H of the datapacket p, the routing module rm_(i) routes the data packet P on thefirst virtual channel. If the destination is greater than its index I,then the routing module rm_(i) routes the data packet P on the secondvirtual channel.

In fact, the choice of a virtual channel corresponds to the choice ofthe corresponding queue for temporary storing the data packet P beforeits transmission on the ring link.

An example of an algorithm implemented in the routing module andillustrating this method is: if ( i = f₂ )  then   route packet P to theNI_(i) direction;  else   if ( i > f₂ )    then     route packet Paccording to f₁ and     on the corresponding first virtual channel;   else     route packet P according to f₁ and     on the correspondingfirst virtual channel;   endif; endif;

The preprocessing modules and the routing modules can be implemented bysoftware within microcontrollers, or can be realized with logic circuitsby using, for example, logic synthesis tools from the algorithmdescribed above and converted in a specific language, like VHDL.

Furthermore, all indexes used in the invention are binary encoded withbits. However, the routing modules need to use, because there is at themost d transitions in a path, only the last ceil(log₂(d)) bits.

This topology of the network associated to a method for routing datapackets permits deadlock problems to be avoided. The resource dependencygraph, in which the virtual channel dependency graph has no cycle, thecorresponding network is deadlock free. The generic scalable networkproposed in this invention has a channel dependency graph cycle free, inparticular, because of the use of two virtual channels peruni-directional ring link and because of the fact that a crossingtransition is always performed before eventual transitions on a circularpath.

Five cases of a data packet transmission according to the invention willbe now illustrated. In FIG. 4, the path of a data packet P from thesource element E₀ to the destination element E₂ is highlighted. Thepreprocessing module ppm₀ of the network interface NI₀ sets the value Rin the first field f₁ of the header H of the data packet P. Then therouting module rm₀ of the router r₀ sends the data packet P on the ringRG in the right or clockwise direction R on the second virtual channelvl2 ₀₁. The router r₁ receives the data packet P. The routing module rm₁of the router r₁ sends the data packet P on the ring RG in the rightdirection R on the second virtual channel vl2 ₁₂. When the data packet Pis received by the routing module rm₂ of the router r₂, this data packetP is transmitted to the network interface NI₂ to the destination elementE₂.

In FIG. 5, the path of a data packet P from the source element E₁ to thedestination element E₉ is highlighted. The preprocessing module ppm₁ ofthe network interface NI₁ sets the value L in the first field f₁ of theheader H of the data packet P. Then the routing module rm₁ of the routerr₁ sends the data packet P on the ring RG in the left orcounter-clockwise direction L on the first virtual channel vl1 ₁₀. Therouter r₀ receives the data packet P. The routing module rm₀ of therouter r₀ sends the data packet P on the ring RG, in the left directionL, on the second virtual channel vl2 ₀₉. When the data packet P isreceived by the routing module rm₉ of the router r₉, this data packet Pis transmitted to the network interface NI₉ to the destination elementE₉.

In FIG. 6, the path of a data packet P from the source element E₈ to thedestination element E₃ is highlighted. The preprocessing module ppm₈ ofthe network interface NI₈ sets the value A in the first field f₁ of theheader H of the data packet P. Then the routing module rm₈ of the routerr₈ sends the data packet P on the crossing link cl₈₃. The router r₃receives the data packet P. The routing module rm₃ of the router r₃ thentransmits the data packet P to the network interface NI₃ to thedestination element E₃.

In FIG. 7, the path of a data packet P from the source element E₁ to thedestination element E₇ is highlighted. The preprocessing module ppm₁ ofthe network interface NI₁ sets the value AR in the first field f₁ of theheader H of the data packet P. Then the routing module rm₁ of the routerr₁ sends the data packet P on the crossing link cl₁₆. The router r₆receives the data packet P. The routing module rm₆ of the router r₆sends the data packet P on the ring RG in the right direction R on thesecond virtual channel vl2 ₆₇. When the data packet P is received by therouting module rm₇ of the router r₇, this data packet P is transmittedto the network interface NI₇ to the destination element E₇.

Finally, in FIG. 8, the path of a data packet P from the source elementE₁ to the destination element E₅ is highlighted. The preprocessingmodule ppm₁ of the network interface NI₁ sets the value AL in the firstfield f₁ of the header H of the data packet P. Then the routing modulerm₁ of the router r₁ sends the data packet P on the crossing link cl₁₆.The router r₆ receives the data packet P. The routing module rm₆ of therouter r₆ sends the data packet P on the ring RG in the left direction Lon the first virtual channel vl1 ₆₅. When the data packet P is receivedby the routing module rm₅ of the router r₅, this data packet P istransmitted to the network interface NI₅ to the destination element E₅.

1-9. (canceled)
 10. A system for routing a data packet between Nelements comprising: N network interfaces respectively connected to theN elements, with N being an even integer; and an on-chip packet-switchedcommunication network arranged in a ring structure comprising N routersrespectively connected to said N network interfaces, N pairs of oppositeuni-directional ring links, with each pair of ring links coupling twoadjacent routers in the ring structure, and each ring link providing twovirtual channels, and N/2 pairs of opposite uni-directional crossinglinks, with each pair of crossing links coupling two diametricallyopposite routers in the ring structure; and processing circuitrydistributed within said N routers and said N network interfaces fordetermining a direction of the data packet to be transmitted over a pathfrom a source element to a destination element in the ring structure,and for determining at each router in the path which virtual channel isto be used.
 11. A system according to claim 10, wherein said processingcircuitry defines the path for the transmitted data packet so that thepath comprises at most d transitions between routers, with d being equalto N/4.
 12. A system according to claim 10, wherein said processingcircuitry defines the path for the transmitted data packet so that thepath includes at least one transition in the ring structure between twoadjacent routers, or a crossing transition between two diametricallyopposite routers followed by a circular path in the ring structurehaving at least one transition, with a direction of the circular pathbeing identical for all transitions in the circular path.
 13. A systemaccording to claim 12, wherein the data packet comprises a headerincluding a first field defining at least one of the crossingtransitions and the direction of the circular path in the ringstructure, and a second field defining a destination router, saidprocessing circuitry comprising N local preprocessing modulesrespectively in said N network interfaces, with one of said localpreprocessing modules being connected to the source element fordetermining contents of the first and second fields; and each routercomprising a routing module for routing the data packet based upon thecontents of the first and second fields.
 14. A system according to claim13, wherein said N routers are successive and are indexed withconsecutive indexes; and wherein a source network interface correspondsto the source element, with said local preprocessing module in thesource network interface comprising: a calculating module forcalculating a value r equal to a difference modulo N between adestination index of the destination router and a source index of thesource router; and a determining module for determining the content ofthe first field from the value r.
 15. A system according to claim 14,wherein the indexes are increased in a clockwise direction; and whereinsaid determining module provides the content of the first field with thefollowing: a first value representative of the clockwise direction inthe ring structure if the value r belongs to the set {1,2, . . . ,d}; asecond value representative of a counter-clockwise direction in the ringstructure if the value r belongs to the set {N−d,N−d+1, . . . ,N−1}; athird value representative of a crossing in the ring direction if thevalue r is equal to N/2; a fourth value representative of the crossingtransition between two diametrically opposite routers followed by thecounter-clockwise direction in the ring structure if the value r belongsto the set {d+1,d+2 . . . ,N/2−1}; or a fifth value representative ofthe crossing transition between two diametrically opposite routersfollowed by the clockwise direction in the ring structure if the value rbelongs to the set {N/2+1,N/2+2, . . . ,N−d−1}.
 16. A system accordingto claim 15, wherein each routing module performs the following: routingthe data packet to its paired network interface if the index of thecorresponding router is equal to the content of the second field of theheader of the data packet; and routing the data packet according to thecontent of the first field of the header of the data packet on a firstvirtual channel if the index is greater than the content of the secondfield of the header of the data packet, otherwise on a second virtualchannel.
 17. A system according to claim 10, wherein each routercomprises at least two queues connected to a uni-directional ring linkconnected thereto, and each virtual channel being defined by at leastone queue and the corresponding uni-directional ring link.
 18. A systemaccording to claim 14, wherein the indexes are binary encoded, and saidrouting modules only process the indexes with the last log₂(d) bits ofthe indexes.
 19. A packet-switched communication network comprising: asubstrate; N network interfaces on said substrate, with N being an eveninteger, and each network interface to be connected to an element; Nrouters arranged in a ring structure on said substrate and respectivelyconnected to said N network interfaces; N pairs of oppositeuni-directional ring links, with each pair of ring links coupling twoadjacent routers in the ring structure, and each ring link providing twovirtual channels; N/2 pairs of opposite uni-directional crossing links,each pair of crossing links coupling two diametrically opposite routersin the ring structure; and processing circuitry distributed within saidN routers and said N network interfaces for determining a path from asource element to a destination element in the ring structure, and fordetermining at each router in the path which virtual channel is to beused.
 20. A packet-switched communication network according to claim 19,wherein said processing circuitry defines the path for the transmitteddata packet so that the path comprises at most d transitions betweenrouters, with d being equal to N/4.
 21. A packet-switched communicationnetwork according to claim 19, wherein said processing circuitry definesthe path for the transmitted data packet so that the path includes atleast one transition in the ring structure between two adjacent routers,or a crossing transition between two diametrically opposite routersfollowed by a circular path in the ring structure having at least onetransition, with a direction of the circular path being identical forall transitions in the circular path.
 22. A packet-switchedcommunication network according to claim 21, wherein the data packetcomprises a header including a first field defining at least one of thecrossing transitions and the direction of the circular path in the ringstructure, and a second field defining a destination router, saidprocessing circuitry comprising N local preprocessing modulesrespectively in said N network interfaces, with one of said localpreprocessing modules being connected to the source element fordetermining contents of the first and second fields; and each routercomprising a routing module for routing the data packet based upon thecontents of the first and second fields.
 23. A packet-switchedcommunication network according to claim 22, wherein said N routers aresuccessive and are indexed with consecutive indexes; and wherein asource network interface corresponds to the source element, with saidlocal preprocessing module in the source network interface comprising: acalculating module for calculating a value r equal to a differencemodulo N between a destination index of the destination router and asource index of the source router; and a determining module fordetermining the content of the first field from the value r.
 24. Apacket-switched communication network according to claim 23, wherein theindexes are increased in a clockwise direction; and wherein saiddetermining module provides the content of the first field with thefollowing: a first value representative of the clockwise direction inthe ring structure if the value r belongs to the a second valuerepresentative of a counter-clockwise direction in the ring structure ifthe value r belongs to the set {N−d,N−d+1, . . . ,N−1}; a third valuerepresentative of a crossing in the ring direction if the value r isequal to N/2; a fourth value representative of the crossing transitionbetween two diametrically opposite routers followed by thecounter-clockwise direction in the ring structure if the value r belongsto the set {d+1,d+2 . . . ,N/2−1}; or a fifth value representative ofthe crossing transition between two diametrically opposite routersfollowed by the clockwise direction in the ring structure if the value rbelongs to the set {N/2+1,N/2+2, . . ,N−d−1}.
 25. A packet-switchedcommunication network according to claim 24, wherein each routing moduleperforms the following: routing the data packet to its paired networkinterface if the index of the corresponding router is equal to thecontent of the second field of the header of the data packet; androuting the data packet according to the content of the first field ofthe header of the data packet on a first virtual channel if the index isgreater than the content of the second field of the header of the datapacket, otherwise on a second virtual channel.
 26. A method foroperating a packet-switched communication network comprising N networkinterfaces on a substrate, with N being an even integer, and eachnetwork interface to be connected to an element; N routers arranged in aring structure on the substrate and respectively connected to the Nnetwork interfaces; N pairs of opposite uni-directional ring links, witheach pair of ring links coupling two adjacent routers in the ringstructure, and each ring link providing two virtual channels; and N/2pairs of opposite uni-directional crossing links, with each pair ofcrossing links coupling two diametrically opposite routers in the ringstructure; the method comprising: determining direction of the datapacket to be transmitted over a path from a source element to adestination element in the ring structure; and determining at eachrouter in the path which virtual channel is to be used.
 27. A methodaccording to claim 26, wherein the path for the transmitted data packetis defined so that the path comprises at most d transitions betweenrouters, with d being equal to N/4.
 28. A method according to claim 26,wherein the path for the transmitted data packet is defined so that thepath includes at least one transition in the ring structure between twoadjacent routers, or a crossing transition between two diametricallyopposite routers followed by a circular path in the ring structurehaving at least one transition, with a direction of the circular pathbeing identical for all transitions in the circular path.
 29. A methodaccording to claim 28, wherein the data packet comprises a headerincluding a first field defining at least one of the crossingtransitions and the direction of the circular path in the ringstructure, and a second field defining a destination router; wherein thepacket-switched communication network further comprises N localpreprocessing modules respectively in the N network interfaces, with oneof the local preprocessing modules being connected to the source elementfor determining contents of the first and second fields; and each routercomprising a routing module for routing the data packet based upon thecontents of the first and second fields.
 30. A method according to claim29, wherein the N routers are successive and are indexed withconsecutive indexes; and wherein a source network interface correspondsto the source element, with the local preprocessing module in the sourcenetwork interface comprising: a calculating module for calculating avalue r equal to a difference modulo N between a destination index ofthe destination router and a source index of the source router; and adetermining module for determining the content of the first field fromthe value r.
 31. A method according to claim 30, wherein the indexes areincreased in a clockwise direction; and wherein the determining moduleprovides the content of the first field with the following: a firstvalue representative of the clockwise direction in the ring structure ifthe value r belongs to the set {1,2, . . . ,d}; a second valuerepresentative of a counter-clockwise direction in the ring structure ifthe value r belongs to the set {N−d,N−d+1, . . . ,N−1}; a third valuerepresentative of a crossing in the ring direction if the value r isequal to N/2; a fourth value representative of the crossing transitionbetween two diametrically opposite routers followed by thecounter-clockwise direction in the ring structure if the value r belongsto the set {d+1,d+2 . . . ,N/2−1}; or a fifth value representative ofthe crossing transition between two diametrically opposite routersfollowed by the clockwise direction in the ring structure if the value rbelongs to the set {N/2+1,N/2+2, . . . ,N−d−1}.
 32. A method accordingto claim 31, wherein each routing module performs the following: routingthe data packet to its paired network interface if the index of thecorresponding router is equal to the content of the second field of theheader of the data packet; and routing the data packet according to thecontent of the first field of the header of the data packet on a firstvirtual channel if the index is greater than the content of the secondfield of the header of the data packet, otherwise on a second virtualchannel.